In the manufacture of integrated circuits, photolithography is used to establish an integrated circuit design on a surface. Important to the continuing development of photolithography are methods which provide for increased chip complexity and decreased size while simultaneously increasing yield and reducing costs.
Briefly, the photolithography process involves the application of a film of light-sensitive polymeric material know as "photoresist" (or simply, "resist") on the wafer surface. A glass mask known as a "reticle" is then positioned above the resist. The reticle has on it the pattern that is to be transcribed into the resist. The pattern consists of features that are opaque to light, portions of the reticle that do not have an opaque feature are transparent. Light is passed through the reticle and is incident on the resist. Due to the opaque features on the reticle, some portions of the resist film are unexposed to light. Thus, the pattern on the reticle is transcribed into the resist in the form of exposed or unexposed regions. The "exposed" resist is then "developed" in a solvent solution. For a +ve (-ve) resist, exposed (unexposed) regions are dissolved away, having a resist pattern on the wafer surface that is a copy of the pattern on the reticle. In subsequent processing steps, the patterned resist acts as a mask for etching patterns into the substrate.
Turning to FIG. 1, when resist 11 is coated on a wafer with an underlying topography, such as step 12, the thickness of the resist varies over the step. As indicated in FIG. 1, the resist's varying thickness causes area A to have a substantially thicker resist coating than area B, and therefore the energy density coupled in the resist is different at these two points. Hence an energy dose which exposes out the resist at point A will cause the resist to be over-exposed at point B, resulting in a variation of the linewidth as it crosses a step.
In order to control light, various anti-reflective techniques have been developed. For example, antireflective coatings (ARCs) are deposited onto a wafer surface and/or on top of one resist layer. Dyes or absorbers have been added to resist itself to control light absorption. Moreover, optical thicknesses have been selected that avoid quarter wavelength multiples of the exposing energy and wavelengths have been mixed to avoid nonuniform intensity distribution in the resist layers.
Other considerations must also be accounted for in suppressing light scattering. For example, surface reflections from different types of substrates vary according to their type and structure. The amount of reflection from both aluminum and silicon varies. Moreover, interfering light from reflections from resist and substrate interference results in light coupling at regular intervals in the resist, causing line-width variations.
An ARC is a thin film, in the range of 100 to 250 nm thick, and is applied to metallization or other highly reflective films onto which submicron imaging scale dimensions are to be placed. These coatings can be applied to within 10 nm coating uniformity when spin-applied. The ARC's application is followed by a spin-applied resist coating. The ARCs presently used are organic materials, both polyimide and non-polyimide based. Moreover, certain inorganic materials (mainly metallic) such as TiN, TaSi.sub.2, TiW, and .alpha.-Si are used.
While the presently used ARCs provide critical dimension control, they cannot be applied with uniform thickness over a step 12, as shown in FIG. 2. The thickness of an ARC's coating 14 varies over the step 12 and therefore, they are susceptible to the effects of undesirable optical phenomenon as well. Furthermore, another disadvantage is that their removal after the etching step requires special processing. While the resist is removed by subjecting it to an oxygen plasma, the ARC is removed by a separate step involving solvents or similar removal substances.